Apatite
Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ion, respectively, in the crystal. The formula of the admixture of the three most common endmembers is written as Ca10(PO4)6(OH,F,Cl)2, and the crystal unit cell formulae of the individual minerals are written as Ca10(PO4)6(OH)2, Ca10(PO4)6F2 and Ca10(PO4)6Cl2.
| Apatite group | |
|---|---|
Apatite (CaF) (fluorapatite) doubly-terminated crystal in calcite | |
| General | |
| Category | Phosphate mineral |
| Formula (repeating unit) | Ca5(PO4)3(F,Cl,OH) |
| IMA symbol | Ap |
| Strunz classification | 8.BN.05 |
| Crystal system | Hexagonal |
| Crystal class | Dipyramidal (6/m) (same H-M symbol) |
| Space group | P63/m (no. 176) |
| Identification | |
| Color | Transparent to translucent, usually green, less often colorless, yellow, blue to violet, pink, brown. |
| Crystal habit | Tabular, prismatic crystals, massive, compact or granular |
| Cleavage | [0001] indistinct, [1010] indistinct |
| Fracture | Conchoidal to uneven |
| Mohs scale hardness | 5 (defining mineral) |
| Luster | Vitreous to subresinous |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 3.16–3.22 |
| Polish luster | Vitreous |
| Optical properties | Double refractive, uniaxial negative |
| Refractive index | 1.634–1.638 (+0.012, −0.006) |
| Birefringence | 0.002–0.008 |
| Pleochroism | Blue stones – strong, blue and yellow to colorless. Other colors are weak to very weak. |
| Dispersion | 0.013 |
| Ultraviolet fluorescence | Yellow stones – purplish-pink, which is stronger in long wave; blue stones – blue to light-blue in both long and short wave; green stones – greenish-yellow, which is stronger in long wave; violet stones – greenish-yellow in long wave, light-purple in short wave. |
The mineral was named apatite by the German geologist Abraham Gottlob Werner in 1786, although the specific mineral he had described was reclassified as fluorapatite in 1860 by the German mineralogist Karl Friedrich August Rammelsberg. Apatite is often mistaken for other minerals. This tendency is reflected in the mineral's name, which is derived from the Greek word ἀπατάω (apatáō), which means to deceive.